The invention provides ink compositions for ink jet printing (ink jet inks), which are highly effective for simultaneously imparting visible and fluorescent images. In the preferred forms, both a dark, visible image and a complementary fluorescent image will be visually discernable as well as machine readable to enable efficient hand and automated processing or handling of the objects printed. The invention achieves these results through the development of ink formulations that moderate the typically occurring phenomenon of quenching while possessing the physical properties necessary for an ink jet ink.
It is generally known to employ automated detectors which are responsive to images with high reflective contrast in the visible region of the spectrum for the machine processing of various types of information-bearing tickets, tags, labels, postage indicia and similar security markings. It is further known to employ automated detectors that are responsive to fluorescent emissions of security markings resulting from excitation at a shorter wavelength such as ultraviolet (UV) excitation. In the postage meter art, for example, mail pieces carrying postage indicia printed with fluorescent ink enhance machine processing. In the United States and Canada automatic equipment correctly faces or orients individual mail pieces by detecting red-fluorescence of postal indicia attached to mail pieces. Postal Service facing equipment employs a simple detector to locate the fluorescence. While useful, detectors of this type do not verify that the fluorescence and the indicium image are physically coincident.
Generally, a fluorescent material fluoresces in a defined region of the spectrum upon exposure to a shorter wavelength excitation light such as UV light. As used herein, the term xe2x80x9cfluorescent security markingxe2x80x9d, refers to such an image. Desirably, the marking will be xe2x80x9cred-fluorescentxe2x80x9d, which term is used herein to refer to fluorescence in the red region of the spectrum as opposed to indicating the visible color of the ink. The shift in wavelength between the incident excitation light and the fluorescent emission clearly distinguishes fluorescence from direct reflection. Fluorescent security markings are effectively applied to detection of forged documents, such as tickets, securities, identification cards, security papers, and the like. The difficulty of copying the fluorescence of security markings deters copying and provides forensic evidence of counterfeits. Among the applications of these security markings are detection of articles, production marking, and automatic article identification. Intensity of the fluorescence is important to the success of these applications. Unfortunately, application of inks by ink jet printing so limits the physical properties of the inks that the normal tendency of the colorants in the ink to quench any fluorescence presents a major technical challenge.
The prior art has provided inks for rotary and other letter press postage meters to imprint indicia on envelopes with platens using ink impregnated into foam or other porous media. Red-fluorescent, colored inks have been made for letterpress meters and include red, blue, green and black inks. For example, U.S. Pat. Nos. 2,681,317, 2,763,785, 3,230,221, 3,560,238, 3,928,226 and 4,015,131 disclose red-fluorescent inks for this purpose. These inks, in general, have non-aqueous, solvent-based vehicle systems with low vapor pressures. Typically, they will have a high solids concentration, a high viscosity, a high boiling temperature and a low surface tension.
Unfortunately, letterpress technology lacks the ability of digital printing to print variable information, and the inks are not useful in ink jet printers, which require stable solutions or dispersions with small particle sizes, low viscosity and a specified surface tension. Special inks must be produced before the many advantages of ink jet printing technology can be realized. They must be low in viscosity and have specific, high surface tension properties to function properly. Beyond that, they must provide high image contrast at low solids concentrations. The viscosity of the liquid ink jet inks is typically from 1.5 to 15 centipoise (cps) in current piezoelectric ink jet printers and about 1 to 5 cps in thermal ink jet printers. A desirable range of surface tension of ink jet printer inks is between 30 to 50 dynes/cm.
These criteria have deterred the development of some fluorescent ink jet inks, but have not permitted any to date that are visually-dark and give high print contrast. A number of red-colored aqueous red-fluorescent inks are disclosed in U.S. Pat. No. 5,681,381 and U.S. Pat. No. 6,176,908, and those inks fulfill United States Postal Service (USPS) requirements for franking while being compatible with use in an ink jet printer. These inks are also stable for extended periods of time. They are based on water, co-solvent and penetrant solutions of water-soluble fluorescent toners. In order to achieve fluorescence with the required fluorescent signal, e.g., phosphor meter unit (PMU), these inks are designed with an optical density lower than that normally required for machine recognition at all wavelengths. These ink formulations are, however, limited in their utilities due to their color and their inapplicability to black or other dark inks.
Postage indicia and franking machines have been developed to make use of digital printing and especially ink jet printing. They have utilized both dark, high-contrast inks and fluorescent inks separately, but no single dark, high-contrast, fluorescent ink has been available. For example, red and purple-colored, red-fluorescent indicia have been printed with variable data using digital printers. Digitally printed indicia provide significant advantages over letterpress indicia. Ink jet printing enables printing indicia with high-density, variable information. Pitney Bowes"" PostPerfect(copyright) meter produces a red-colored red-fluorescent indicium with variable data by thermal transfer printing while the Personal Post Office(trademark) system produces red-colored, red-fluorescent indicia by ink jet printing. The USPS xe2x80x9cInformation-Based Indicia Programxe2x80x9d (IBIP) allows the use of a black machine-readable indicia. The Post Office equipment typically orients mail pieces bearing IBIP indicia using a facing identification mark (FIM) or by fluorescent tags added to the indicia. However, because there is no fluorescent black ink available for ink jet printing and using an FIM printed at the edge of an envelope is difficult, the IBIP use is limited.
Postal services utilize machine-readable variable information for a variety of value-added services, for cryptographic authentication of the indicium and for obtaining marketing information. Compared to letterpress technology, digital printers can provide print quality and contrast that do not decrease with the number of prints. The images can be printed at high resolution, with high quality and at high speeds by direct, non-impact print engines. These inks have additional advantages for security markings since they may include penetrating solvents that cause the selective penetration of colorant into the paper. This penetration provides rubbing and scratch resistance to the security markings. Unfortunately, the use of ink jet printing for postage franking is restricted, to some extent, by the present lack of inks functional with ink jet technology that are simultaneously suitable for franking and machine-readability.
Information contained in printed indicia is useful for security and marketing purposes, as well as for processing the mail. In particular, the IBIP contains high-density variable cryptographically protected information in a two-dimensional bar code. To capture this information, postal scanning equipment must efficiently detect and read the information-based indicium. Postal indicia must display sufficient contrast in reflection to enable machine-readability, regardless of the substrate. However, available red-fluorescent inks tend to exhibit low contrast, inhibiting their ability to be reliably read by optical character recognition (OCR) equipment, bar code readers and other types of machine vision technology. These systems often have illumination and detection systems in the red region of the spectrum, limited by laser systems. The substrate can also limit machine readability. On dark substrates, such as Kraft envelopes with a reflectance of between 0.45 and 0.6, it is very difficult to achieve sufficient contrast with red inks. Therefore, there is a strong need for printing security markings which exhibit high contrast, preferably black, and simultaneously fluorescence, particularly red-fluorescence.
Another challenge to the achievement of inks for security features having practical utility is that there are a large variety of commercially-available organic luminescent compounds that might confuse security systems based on currently-available inks. Common examples of these organic luminescent compounds are the optical brighteners and commercially available colored-fluorescent materials and inksxe2x80x94all lighter-colored inks. These might permit fraudulent replication of indicia, e.g., printed in red or green, by substitution of a luminescent substance that emits light of a similar color for an authentic material. This type of normally-available organic luminescent compound could not provide visually-dark and red-fluorescent images. It is, however, another reason why it would be advantageous to provide fluorescent inks with unique optical properties that cannot be easily simulated with materials that are readily available.
The achievement of suitable ink jet inks with suitable physical and fluorescing characteristics presents a major technical challenge because of the physical characteristics required of the fluid ink and a typical fluorescence phenomenon known in the art as quenching. Thus, there are technical reasons why dark, fluorescent inks are not available. The problem of quenching will be explained briefly below.
In the fluorescence process, the absorption of a light quantum by a molecule brings it to an excited singlet state. The time of absorption is about 10xe2x88x9215 seconds. From the excited, singlet state, light is emitted to the ground level as fluorescence. The 10xe2x88x929 second duration of the fluorescence process is much longer than the absorption process. Four separate processes affect the observed fluorescence. In one, not quenching, competing light absorption from other dyes can reduce the observed fluorescence due to less light being absorbed by the fluorescent dye. In another, (xe2x80x9ctrivial mechanismxe2x80x9d of quenching) absorption by other dyes of the light emitted by the fluorescent dye will reduce the observed fluorescence. In the third, quenching can occur by collisional energy transfer between the excited fluorescent dye molecule and a non-fluorescent dye molecule when the two come into close contact. The fourth mechanism, called resonance energy transfer, does not involve contact of the two molecules and can occur over a considerable distance.
The lifetime of a fluorophore can be related to the concentration of a quencher by the Stern Volmer equation, xcfx840/xcfx84=1+Ksv(Q), where xcfx840 is the lifetime of the fluorophore in the absence of the quencher, Ksv is the lifetime of the fluorophore in the presence of the quencher, Ksv is the Stern Volmer constant and Q is the quencher concentration. As the concentration of the quencher increases, the excited state of the fluorophore is quenched causing a reduction in the lifetime.
One quenching mechanism is transfer of the energy absorbed by a donor molecule to an acceptor molecule. Unless the acceptor molecule is a fluorophore, i.e., a fluorescent dye, the energy transfer process will deactivate the excited state and quench the fluorescence. If the acceptor is a fluorophore, the energy transfer can excite the acceptor, which then fluoresces at a longer wavelength. This process of donors fluorescing in the short-wavelength, visible region of the spectrum while the absorption spectra of the acceptor overlaps the emission spectra of the donor and, as a result, the acceptor fluorophores fluoresce more strongly at longer wavelengths, is known as cascading. The selection of a mixture can also result in the absorption spectrum of the acceptor overlapping the fluorescence spectrum of the donor. In such a case, the resultant effect is the sensitization or enhancement of the light emission of the acceptor.
In order to achieve a conventional black ink based on water-soluble dyes, a single dye or a mixture of dyes is required which will absorb across the entire visible spectrum, from 390 nm to ca. 680 nm. If a single dye is to be used, it must exhibit very broad absorption and/or multiple visible absorption bands. If mixed dye systems are to be used, this would require at least two dyes (orange and violet with broad absorption bands), or more usually three dyes (e.g. yellow, purple and blue). Such a mixed black system would not normally show red-fluorescence, both because of competition among the various dye components for the UV light and because of efficient quenching of the fluorescence. The quenching results from energy transfer to those non-fluorescent dyes in the composition that have absorption bands overlapping with the emission band of the fluorescent dye, most significantly to the blue dye component of the mixture. Such energy transfer could occur by collisional transfer where the non-fluorescent acceptor diffuses to the donor, at a distance by resonance transfer, or by the so-called xe2x80x9ctrivialxe2x80x9d mechanism whereby the blue dye absorbs any red emitted light. Similar problems exist with available inks based on the use of one or more pigments or dyes.
From the above discussion, it can be seen that currently-available inks for ink jet printing cannot provide high-contrast visible images as well as fluorescent images suitable for security markings. There remains a technical challenge to the provision of such inks, which would be highly desirable if available.
Thus, it is an object of this invention to provide a photosensitive optically-variable, e.g., fluorescent, ink jet ink which produces a high contrast, e.g., machine-readable, image in reflection.
Another object of the invention is to provide a means to inhibit the normal quenching of fluorescence that deters the production of a useful a fluorescent ink jet ink capable of producing a high contrast machine-readable image in reflection.
It is another object of the invention to provide a multi-component ink formulation of the type described with components that differentially adhere to the paper substrate, thus making alteration or non-destructive transfer to a counterfeit document extremely difficult.
It is another object of the invention to provide fluorescent inks with unique optical properties that cannot be easily simulated with materials that are readily available.
It is yet another object of the invention to provide an ink of the type described useful as a forensic verifier in that it can verify that the dark regions and the fluorescent regions of a security marking are coincident.
These and other objects are achieved by the invention which provides inks suitable for ink jet printing, a process for preparing such inks, a printing process using the inks and printed substrates bearing images printed with the inks.
In one aspect, an ink of the invention will be defined as a homogeneous, aqueous ink capable of producing dark, machine-readable markings exhibiting fluorescence when exposed to fluorescent-exciting radiation, the ink being of suitable viscosity and surface tension for use in ink jet printing, comprising: (a) a first colorant comprising at least one fluorescent dye, said fluorescent dye emitting light within a characteristic emission band when excited by fluorescent-exciting radiation; (b) a second colorant having a light absorption band at longer wavelengths than the characteristic emission band of the first colorant, said second colorant comprising a water-soluble polymeric dye of effective molecular configuration to inhibit quenching of fluorescence of said first colorant due to collisional energy transfer with said second colorant; and (c) an aqueous liquid vehicle comprising water and a water-soluble vehicle in sufficient amounts to achieve an ink viscosity and surface tension effective for application of the ink to a substrate in a predetermined pattern by ink jet printing; wherein, the colorants are present in combination in the aqueous ink in amounts effective to cause the ink, when dry, to exhibit a dark color due to the net absorption spectra of the colorants in the visual range and machine-readable or visually-discernable fluorescence when subjected to fluorescent-exciting radiation. Desirably, within the spectral range of interest of from about 390 to 680 nm, the ink reflectance is less than 50% of the paper reflectance.
In preferred ink formulations, a fluorescence stabilizer, i.e., a material capable of limiting the loss of fluorescence due to migration of the ink into a porous substrate, will also be employed. Also preferred is the use of surfactants to adjust physical properties. Many other preferred and alternative aspects of the invention are described below.